The present invention relates in general to acousto-optic scanners, and is particularly directed to an arrangement and method for controlling pixel clock and scan timing of an acousto-optic scanner, in accordance with the acoustic velocity of an acoustic wave lens (ATWL) traveling through the scanner""s acousto-optic waveguide.
A number of industrial systems for conducting extremely high resolution optical scanning of a workpiece, such as a semiconductor substrate, may employ an acousto-optic Bragg cell-based scanner. Critical to success of operation of such systems is the need for extreme precision in the alignment of the light beam and the workpiece. An acoustic traveling wave lens (ATWL) scanner is capable of providing such position accuracy while scanning at very high speed. This positioning accuracy of the ATWL scanner is derived from the fact that the scan progresses with a traveling acoustic wave in a highly stable material, such as fused silica.
Fused silica has excellent dimensional stability due to its low thermal expansion coefficient of 0.6xc3x9710xe2x88x926 per degree Centigrade. However, the variation in the acoustic velocity of fused silica is much higher. The temperature coefficient for scan velocity is near 1xc3x9710xe2x88x924 per degree Centigrade. As a consequence, the principal placement error in an ATWL scanner arises from the change in acoustic velocity with temperature. It is common practice to vary the sampling time during a scan, in order to compensate for scanner placement errors. Namely, the time of taking or exposing samples is varied in such a way as to cause the samples to occur in the proper place on spatial sampling grid.
The present invention is directed to a new and improved apparatus and method for generating a pixel clock for an ATWL-based optical scanner, wherein the pixel clock is varied in such a manner to provide a uniform and constant sampling grid, independently of small acoustic velocity variations in the ATWL propagation medium. As will be described in detail below, the pixel clock is derived as a function of the propagation velocity in the ATWL medium in such a manner to render each pixel spatially invariant to propagation velocity changes in the ATWL medium. This means that as changes in temperature retard or increase the speed of the pressure-induced lens traveling from the excitation transducer to the end of the ATWL cell on each scan, the pixel clock is correspondingly slowed down or speeded up by the same proportional amount, so as to maintain registration in time and space. The compensation mechanism employed by the present invention measures the time it takes for the pressure induced lens to travel the length (or large portion thereof) of the ATWL cell. It then forces the pixel rate to produce a desired number of pixels within the same time interval.
In accordance with the invention, scanner system timing is governed by an acoustic velocity-driven, phase locked loop containing an adjustable voltage controlled pixel clock generator (VCXO), which is controlled by a detector that produces delayed and attenuated replica of the excitation waveform applied to an ATWL scanner used to scan a light beam across a workpiece. Pursuant to a first embodiment of the invention, an end-of-cell transducer converts the pressure induced traveling lens into an electrical signal replica of the excitation input. In a second embodiment, an end-of-scan optical pick-off monitor is employed to detect the scanned optical spot as it crosses its field of view.
The pixel rate clock signal is used to locate spatially repeatable time instances along the optical scan of the ATWL scanner. The pixel rate clock signal is coupled to a subharmonic rate generator, which outputs a relatively low rate clock signal having pixel registration edges, so as to facilitate scan cycle timing events through use of commercially available logic devices. The reduced rate clock signal is used to clock a cycle timing generator, programmable delay line, a set of cascaded flip-flops, and an up/down counter that drives a digital-to-analog converter (DAC) whose output of which is used to adjust the clock rate of the VCXO.
Using the low rate clock signal produced by the subharmonic rate generator, the cycle timing generator initiates a scan cycle and all subsequent scan events, in response to an externally sourced scan request strobe. This programmable delay line provides for fine tuning of the pixel rate VCXO about its nominal center of range of operation, and thereby allows the full VCXO range of to be applied to pixel rate compensation due to temperature-induced propagation velocity changes experienced by the ATWL scanner under normal operating conditions. Also the small fixed propagation delays associated with various other devices, cables, filters, etc., are readily removed using the programmable delay line.
The output of the delay line is coupled to a direct digital synthesis (DDS) based five-cycle burst generator that is enabled for a prescribed number of subharmonic cycles (beginning at a first selected ATWL gate clock count terminating at a second clock count of the output of the cycle timing generator. This produces a scanner excitation waveform comprised of a fixed plurality of cycles of a reduced clock signal. This excitation waveform is low pass filtered to produce a filtered burst signal that is amplified and applied to the ATWL scanner by way of an input transducer.
The acoustic traveling wave lens scanner cell may comprise of section of optical material that supports the propagation of a pressure wave (or series of acoustic waves) with low attenuation along its length from the input transducer to an output detector (end-of-cell output transducer in the first embodiment, end-of-scan optical monitor in the second embodiment). The traveling pressure wave creates a lens that provides a relatively high spot resolution of the deflected optic beam aligned with the lens as it travels the length of the ATWL cell.
A buffer amplifier and zero-crossing comparator coupled to the scanner detector output amplify the output signal derived from the ATWL scanner and convert the attenuated, delayed replica of the scanner""s excitation signal into logic levels that are sampled for polarity. The first of the two cascaded flip-flops monitors the output of the zero-crossing comparator, while the second flip-flop monitors the output of the first flip-flop. The first flip-flop determines if a selected (e.g., third negative-going) zero-crossing of the delayed excitation replica of the scanner""s input burst signal occurs early or late, relative to a particular pixel subharmonic cycle""s leading edge, and samples and stores this decision. The second flip-flop reduces the probability of a metastable output being coupled to the up/down counter.
The up/down counter increments on a relatively later (e.g. 298th) subharmonic cycle produced by the cycle timing generator, if the digital input to the counter from the second flip-flop is a logical xe2x80x9czeroxe2x80x9dxe2x80x94indicating that third negative-going zero crossing of the delayed replica of the scanner""s excitation burst had already occurred prior to a slightly earlier (e.g., 296th) subharmonic cycle sampling the first flip-flop. Conversely, loading xe2x80x9conexe2x80x9d is loaded into the first flip-flop on the 296th subharmonic cycle indicates that the pixel clock rate is too fast, since the leading edge of the 296th subharmonic cycle occurred before the third cycle""s negative going zero-crossing of the excitation replica (i.e., the subharmonic cycle was early) and the up/down counter is decremented by one. The DAC produces an analog voltage proportional to the digital count stored in the up/down counter. At a minimum count, the DAC generates a voltage to drive the VCXO to its minimum output frequency (300 MHzxe2x88x92200 ppm), while at a maximum count it generates a voltage to drive the VCXO to a maximum output frequency (300 MHz+200 ppm).
The servo-mechanism of the invention operates to align the 296th pixel subharmonic cycle""s leading edge with the third negative going zero-crossing of the replica of the scanner""s excitation burst applied to the ATWL scanner and derived by the scanner""s output device in response to the resultant propagating pressure waves through the cell. The excitation waveform applied to the ATWL scanner is initiated with the tenth pixel subharmonic cycle of the scan, so that to a first order approximation, there are 286 subharmonic cycles between excitation initiation and the third negative going zero-crossing of the excitation replica as the traveling lens exits the ATWL scanner.
As the propagation velocity of the acoustic wave lens traveling through the scanner varies with temperature changes in the ATWL cell, the pixel rate is changed to maintain the alignment of the 296th pixel subharmonic cycle""s leading edge with the third negative going zero-crossing of the excitation replica. Tracking resolution is determined by the dynamic range of the DAC and the up/down counter. Dispersion effects in comparator outputs, D-flip-flop set-up boundaries, and the pixel rate divider contribute a dynamic timing error which is introduced as averaged low level xe2x80x9cnoise.xe2x80x9d This averaging is accomplished by employing very fine steps (e.g., on the order of one picosecond) and assuming dispersion time-invariance from scan to scan. Pixel clock registration with the traveling lens is limited by the pulling range of the pixel VCXO, so that propagation velocity changes within the ATWL cell are limited to the pulling range. This can be accomplished by regulating the ATWL temperature to within xc2x12xc2x0 C., if a quartz is used as the material for the ATWL cell.
In the second embodiment, scanner system timing is controlled by an end-of-scan optical monitor, a narrow optical aperture element is coupled to a high pass filter, which behaves as a time differentiator to augment the end-of-scan spot""s time-of-arrival decision thresholding. If the end-of-scan optical aperture is sufficiently narrow, the spot illumination captured by the optical detector will be representative of the spot profile in time. By using a time differentiator, the zero-crossing comparator of the first embodiment may be employed, and a determination can be better made of the center of the spot rather than of one edge or the other.
If the monitoring aperture is not sufficiently small, the scanned spot will tend to produce a xe2x80x9cflat topxe2x80x9d response, while most of the spot falls within the monitoring aperture. In this instance, a zero-crossing comparator cannot be used, since the time differentiator will produce a positive response, as the positive leading edge is experienced that will return to zero as the xe2x80x9cflatxe2x80x9d response ensues. It will produce a negative response as the spot""s negative-going trailing edge is experienced, which will return to zero after the scanned spot leaves the aperture. Hence, the zero-crossing detection will not be activated properly, since a xe2x80x98decisivexe2x80x99 zero-crossing event does not occur.
To obviate this potential problem, a xe2x80x9cknife-edgexe2x80x9d element can be placed towards the far side of the active detector area, so that a spot amplitude estimate can be made, and half this value can be used as a decision crossing threshold by a comparator, as the midpoint of the spot encounters the xe2x80x9cknife edgexe2x80x9d element. Placing the xe2x80x9cknife edgexe2x80x9d element towards the far side of the active sensitivity area of the optical detector also allows the initial response of this mid-value comparator to stabilize, xe2x80x9clongxe2x80x9d before the desired pixel subharmonic cycle""s leading edge samples the comparator output, to achieve alignment with the (negative) threshold crossing, produced as the xe2x80x9cknife-edgexe2x80x9d blocks the midpoint of the spot.
Each embodiment shares a number of common aspects that enable the functionality of the invention to be realized. The first involves the use of a divided down subharmonic (e.g., divide by eight or ten) that retains every fine time phasing with a high pixel rate clock (e.g., 300 MHz), since it is not necessary to generate all the scan cycle event timing at the high pixel rate; instead it is more cost effective to perform these with the use of CMOS or TTL elements at 50 MHz or less. The only restriction this imposes is to require that events occur on the subharmonic edge boundaries. The primary events include the initiation of the ATWL excitation and the desired number of integer subharmonic cycles that exist between excitation and either a particular zero-crossing end-of-cell transducer signal waveform or the end-of-scan optical monitor experiencing the scanned spot in its field of view.
A second feature involves how the treatment of time/phase error. In both embodiments, a single early/late decision is made at the scan. The pixel rate is then changed by only a very small amount in the direction to reduce the time/phase error. The next early/late decision will either result in another change in the same direction as before and by the same small amount, or it will reverse the effect of the previous early/late decision. This produces a tracking effect with inherent filtering.
A third aspect of the invention obviates the need for a start of scan optical pick-off to provide pixel registration at the workpiece. Instead, start of scan is provided as a single, gated pulse at the pixel subharmonic rate, which occurs a desired number subharmonic cycle intervals prior to the early/late decision event.